With the objective of determining the extent of participation of nutrient transport in cellular growth control, we have isolated membrane vesicles from fibroblasts (3T3, SV3T3, CEF) in culture. These membranes transport phosphate by a saturable process. They also demonstrate sodium-gradient stimulated alpha-amino isobutyric acid uptake. With membrane vesicles isolated from normal and transformed cells we shall compare the molecular mechanisms of transport to determine those changes which occur upon neoplastic transformation. Membranes isolated from temperature-sensitive transformed cells at various stages of transformation will be studied. Our studies will include a correlation of the effect of serum and purified growth factors on the isolated transport system with the growth stimulatory effect of serum on whole cells. We shall investigate the mechanisms by which these growth stimulators affect the transport of nutrients by the cell surface.